In the laser drive circuits of electrophotographic image forming apparatuses, a method of detecting the output of laser light in a light-detection zone of one scan and holding laser driving current of one scan is used to hold a fixed quantity of laser light of one scan.
Referring to FIG. 7, the following will describe a specific control method. In such image forming apparatuses, as shown in FIG. 7, a laser chip 43 constituted by one laser 43A and one photodiode (hereinafter, referred to as PD) sensor 43B is used. By using two current sources of a source 41 of bias current and a source 42 of pulsed current are used for the laser 43, the light-emitting characteristic of the laser 43A is improved. Further, in order to stabilize the light emission of the laser 43A, feedback is given to the source 41 of bias current by using an output signal from the PD (photodetector) sensor 43B and the amount of bias current is automatically controlled. That is, a logic element 40 outputs an ON signal to a switch 49 in response to a signal of lighting at full power from a sequence controller 47, so that the laser 43 is fed with the sum of the current from the source 41 of bias current and the source 42 of pulsed current. The output signal from the PD sensor 43B at that time is inputted to an I/V converter 44, amplified by an amplifier 45, and inputted as a signal VPD to an APC circuit 46. Further, the output signal is supplied as a control signal VAPC from the APC circuit 46 to the source 41 of bias current. This circuit system is called an APC (abbreviation of Auto Power Control) circuit system which is currently a typical circuit system for driving a laser. The laser has a temperature characteristic. The higher temperature is, the larger amount of current for obtaining a fixed quantity of light increases. Since the laser generates heat by itself, a fixed quantity of light cannot be obtained only by supplying a constant current. In a solution to this problem, by using the APC circuit system for each scan, the amount of current applied to the laser diode 43A is controlled to a fixed amount such that each scan has a constant laser light-emitting characteristic. Generally in electrophotography, laser light is detected by a sensor disposed on a fixed position to generate a synchronizing signal of main scan, and a BD (beam detect) signal is generated. Thus, for example, a full power output period of laser is set from a position immediately before the sensor for BD is radiated with laser light to the irradiation of the laser light on a photoconductive drum; meanwhile, APC control is performed such that the source 41 of bias current is controlled by the APC circuit 46 to have a fixed quantity of output light for each scan. A bias current determined in the full power output period is kept during one scan.
The laser light controlled with a fixed light quantity is blinked by turning on/off the switch 49 according to data modulated by a pixel modulating unit 48; meanwhile, an electrostatic drum is scanned and an image is formed thereon.
Further, in the case of a multilaser such as a 2-beam laser, only one PD is provided for a plurality of lasers in the structures of the lasers. Thus, time-division APC is performed in which a laser on one side is turned on during one scan and the light quantity of the laser is monitored by the PD to perform APC control. After the laser is turned off, the other laser is turned on and the light quantity of the laser is monitored by the PD to perform APC control.
Even when the quantity of laser light is uniformly controlled thus by APC control, some characteristics of a used photoconductor may cause a considerably unevenness of electric potential (ΔE>5V) in the main scanning direction as shown in FIG. 8. The unevenness of electric potential is caused by difficulty in the manufacturing of a thin film. Regarding a thin film formed on a surface of a photoconductive drum, it is extremely difficult to have an even thickness in the main scanning direction as compared with the circumferential direction. By using a photoconductor having a thin film with an uneven thickness in the main scanning direction, an exposed part has an unevenness of electric potential on some positions in the main scanning direction, resulting in a longitudinal unevenness of density along the sub-scanning direction in an image. Therefore, image quality seriously decreases.
In the case of an image forming apparatus using a multi-beam laser such as a 2-beam laser, as shown in FIG. 10, illumination distributions obtained by lasers on a drum are different from each other due to the far field patterns of the lasers, a distance between two beams, a state of attachment to an optical unit, and so on. Hence, an unevenness of density appears in the front and rear sides of an image and seriously degrades image quality.
In order to solve these problems, the following technique is devised: light quantity data obtained on two or more points of one scanning line with a laser is measured and stored, the light quantity data on each point is subjected to D/A conversion, and a laser driving current is controlled in response to a signal obtained by filtering an analog signal, which has been obtained after D/A conversion, through a low-pass filter (Japanese Patent Laid-Open No. 2004-223716).
In this method, when the number of measured points is reduced to have a smaller storage capacity, a somewhat smooth light quantity distribution shown in FIGS. 8 and 10 can be corrected by reducing the cut-off frequency of the low-pass filter. However, when a light quantity distribution with a rapid change shown in FIGS. 9 and 11 is corrected, the low-pass filter becomes poor in tracking and thus the light quantity distribution is insufficiently corrected. When the cut-off frequency of the low-pass filter is raised by increasing the number of measured points, the low-pass filter is improved in tracking, and thus it is possible to correct the light quality distribution with the rapid change shown in FIGS. 9 and 11. However, a storage capacity for storing measurement data is increased, resulting in a problem of cost.